Elastin is an amorphous protein present in the elastic fibers of tissues such as arteries, blood vessels, skin, tendons and elastic ligaments, the abdominal wall, and lungs. Unlike other fibrous tissues like collagen, Elastin is unique in that it may be stretched to over 150 percent of its original length but it can rapidly return to its original size and shape. This property of Elastin provides tissues that incorporate it, the required ability to resume their original form after stretching due to blood flow, breathing, or bending. Like collagen protein, Elastin contains about 30% glycine amino acid residues and is rich in proline. Elastin differs from collagen in that it contains very little hydroxyproline and no hydroxylysine. Elastin has a very high content of alanine and also contains two unique amino acids isodesmosine and desmosine. These amino acids are believed to be responsible for Elastin's ability to return to its original shape after stretching.
Tropoelastin is a soluble precursor of Elastin; it is a peptide with a molecular weight in the range of 70-75 kDa. In the arterial tissues tropoelastin is produced and secreted into the extracellular space by smooth muscle cells; in other tissues it is produced in cells, like fibroblast cells, and is also secreted into the extracellular space. In these cells tropoelastin is synthesized by ribosomes in the rough endoplasmatic reticulum and processed by the Golgi apparatus. The soluble tropoelastin molecules secreted (often referred to a Proelastin before secretion) into the extracellular space synthesize to form Elastin filaments and sheets via cross linking of the tropoelastin molecules primarily by crosslinking of lysine amino acid residues to form desmosine and isodesmosine. Mature Elastin is amorphous and contains many cross links which makes it nearly impossible to solubilize.
The resiliency of skin is maintained by elastic fibers in the extracellular matrix (ECM). These ECM components are organized into a networks of rope-like structures and composed of two major components: an amorphous core, consisting of extensively crosslinked elastin which makes up the bulk (>90%) of the fiber; and the 10-12-nm microfibrils made up of several distinct glycoproteins.
In various tissue or biological functions, inelastic collagen fibers may be interwoven with the Elastin to limit stretching of the Elastin and prevent tearing of Elastin comprising tissue. Elastic fibers may also contain glycoproteins as microfibrils, which may serve to organize tropoelastin molecules secreted into the extracellular space for later crosslinking. Examples of such glycoproteins include laminin, which is large glycoprotein and a major component of basement membranes and is made by all epithelial cells, and fibronectin which is a cell-surface and blood glycoprotein involved in a variety of cell surface phenomena.
Combinations of components of the extracellular matrix have been incorporated into cosmetic compositions. Elastin is insoluble due to its high degree of cross linking at its lysine residues and also because of its high content (about 75%) of hydrophobic amino acids (Gly, Val, Ala, Pro). In some instances, normally cross-linked insoluble Elastin (i.e., insoluble in water, organic solvents, and physiological fluids such as saline and blood) is rendered soluble using a variety of chemical and enzymatic methods to cleave insoluble Elastin protein and form smaller peptide fragments.
The human skin consists of two layers; a superficial layer called the epidermis which is epithelial tissue and a deeper layer called the dermis that is primarily connective tissue. These two layers are bound together to form skin which varies in thickness from less than about 0.5 mm, to 3 or even 4 millimeters. The connective tissue found in skin is essentially an intricate meshwork of interacting, extracellular molecules that constitute the so-called “extracellular matrix”. The extracellular matrix includes proteins that are secreted locally and are widely distributed in the extracellular matrix. The main types of proteins that make up the matrix include collagens, Elastin, fibronectin and laminin. Normal elastic fiber assembly is visualized as a spider web spanning the dermis. Exposure of the skin to ultraviolet and visible light from the sun, wind, and chemicals leads to loss of moisture in the epidermal layers and degradation of the Elastins present in the skin. Loss of elasticity in skin primarily occurs because of an overproduction of poorly assembled elastic fibers induced by exposure to sunlight. These poorly assembled elastic fibers can be visualized as “clumps” in the dermoepidermal junction and papillary dermis and is commonly referred to as solar elastosis. These effects, result in loss of skin elasticity, tone and texture, are collectively referred to as aging of the skin. Loss of elasticity in elastic tissues such as arteries is mainly due to calcification and glycation of elastic fibers.
Until recently, elastin, the major component of elastic fibers, was thought to have primarily a mechanical role in providing tissue resiliency. This view was challenged by results of in vitro studies indicating that soluble fragments of tropoelastin and elastin degradation products may bind to the cell surface Elastin Binding Protein (EBP) and stimulate proliferation and migration of human skin fibroblasts, lymphoblasts, smooth muscle cells and cancer cells.